Chemical and biological sensors that are used for continuous monitoring generally require a degree of inertness from the sample environment. Inertness is particularly important in utility type applications such as water monitoring. Continually operating sensors in the field need to be rugged, chemically stable and readily manufactured. There is a continuing need to prepare and design sensors that can effectively and efficiently measure a wide range of chemical and biological contaminants in drinking water in view of both public health and national safety perspectives.
The low salt content (high electrical impedance) of drinking water presents a unique challenge to electrochemical measurement because small variations in electrolyte content will introduce significant measurement error. Improvements in the ability to measure variable conductivity water samples from drinking water to sea water without analytical performance degradation are presently needed. Even gold or gold-coated electrodes are known to degrade in such environments. Accordingly, improvements in electrodes and sensors are required.
U.S. Pat. No. 6,905,655 to Gabriel et al. discloses sensors that operate on the principle that the electrical conductivity of a MWNT changes depending on the environment surrounding nanotube. The disclosed sensors, however, require one to carefully lay down MWNTs (“CNTs”) parallel to the surface of a substrate. Nanotubes oriented in such a fashion are required to make electrical contact with two or more electrodes on the substrate through the outer surface of its graphene sheet. Such sensors typically require that the nanotubes are bonded with some type of protective coating, such as a polymer, where the nanotubes contact the electrodes. In view of the difficulty of adhering nanotubes lying across electrodes in this fashion, there remains the need to provide CNT-based sensors that overcome these difficulties.
Li et al., Nano Letters, 2003, Vol. 3, No. 5, 597-602, discloses a carbon nanotube electrode array for ultrasensitive DNA detection. The nanoelectrode array is based on multiwalled carbon nanotubes embedded in SiO2, with only the open ends of the multiwalled carbon nanotubes being exposed to the environment to give rise to DNA detection. Accordingly, only a very low surface area is provided in the carbon nanotube electrode arrays provided by Li et al. Further improvements are needed to enhance the sensitivity of carbon nanotube electrodes and sensors.